The convert command does many conversions (see Section 10.1.10). Among other things, it can convert between a polynomial in internal sparse multivariate format and a list representing the polynomial.
| p:=symb2poly(x^2-x*y+2y,[x,y]) | 
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| l:=convert(p,list) | 
or:
| l:=convert(p) | 
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which is a list of the coefficients followed by a list of the variable powers.
| convert(l,polynom) | 
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